# Nickel Tetra-(4-Sulfonatophenyl) Porphyrin/Ionic Liquid Supramolecular Assemblies for Applications in Symmetrical Aqueous Redox Flow Batteries

## TL;DR

This paper explores using a nickel porphyrin molecule in redox flow batteries to simplify design and improve performance.

## Contribution

The study introduces a nickel tetra-(4-sulfonatophenyl)porphyrin as a dual-function redox-active molecule for aqueous organic redox flow batteries.

## Key findings

- Nickel tetra-(4-sulfonatophenyl)porphyrin serves as both an anolyte and catholyte in redox flow batteries.
- The ionic liquid BupyBF4 enhances stability and electrochemical performance of the system.
- DFT calculations confirm improved chemical and electrochemical stability with metal insertion.

## Abstract

Redox flow batteries
(RFBs) are a promising technology
as a grid-level
energy storage system and have attracted a growing amount of attention.
In these devices, electrochemical storage is carried out through the
reduction and oxidation of chemical species. The peculiarity of RFBs
is that active species are in solutions, with the reaction occurring
at the solid–liquid interface. In the present work, we propose
the use of nickel tetra-(4-sulfonatophenyl)­porphyrin (NiTPPS) as an
innovative bipolar redox-active molecule (BRM) for aqueous organic
redox flow batteries (AORFBs). Thanks to its distinctive redox properties,
this single yet complex molecule serves as both an anolyte and a catholyte.
This symmetry allows RFBs to use identical components, offering simplified
storage and reduced crossover benefits. To increase NiTPPS stability
in aqueous solution, we explored the ionic liquid (IL) 1-butylpyridinium
tetrafluoroborate (BupyBF4) as a supporting electrolyte
for AORFBs (Huang et al., 2019). Using an IL was also advantageous
in broadening the water electrochemical potential window. Actually,
by DFT calculations and aggregation studies, carried out by UV–vis
spectroscopy, it was observed that the insertion of the metal atom
enhances the chemical and electrochemical stability of the porphyrin
macrocycle. In addition, the use of BupyBF4 as a supporting
electrolyte improved the resolution of redox processes, avoiding problems
associated with water electrolysis and demetalation of the electroactive
species.